Gunn diode, NRD guide gunn oscillator, fabricating method of gunn diode and structure for assembly of the same

ABSTRACT

A Gunn diode which is formed by sequentially laminating a first semiconductor layer, an active layer and a second semiconductor layer onto a semiconductor substrate. The Gunn diode comprises first and second electrodes arranged on the second semiconductor layer for impressing voltage on the active layer, and a concave portion which is cut from around the first electrode in a direction of the second semiconductor layer and the active layer and which subdivides the second semiconductor layer and the active layer to which the first electrode is connected as a region which functions as a Gunn diode. Since etching for defining a region that is to function as a Gunn diode is performed by self-alignment dry etching utilizing electrode layers formed above this region as masks, variations in characteristics are restricted. There are also disclosed a NRD guide Gunn oscillator attached to the NRD guide for obtaining a high frequency osclllation output of the Gunn diode, a fabricating method of the Gunn diode, and a structure for assembly of the Gunn diode.

BACKGROUND OF THE INVENTION

The present invention relates to Gunn diodes used for oscillation ofmicrowaves and millimeter waves, and is especially related to Gunndiodes which realize improvements in thermal characteristics, yieldfactor of good products and easy assembly to planar circuits,fabricating methods thereof and structures for assembly of the same.

The present invention also relates to NRD guide Gunn oscillators thatare comprised by combining a NRD guide (Non Radiative DielectricWaveguide) circuit and Gunn diodes.

Gunn diodes for oscillation of microwaves or millimeter waves areusually comprised of compound semiconductors such as gallium arsenide(GaAs) or indium phosphide. It is the case with such compoundsemiconductors that the electron mobility is several thousands ofcm²/V·sec and thus large in a low electric field while the mobility isdecreased in case a large electric field is applied since acceleratedelectrodes transit to a band of large effective mass and thus causesgeneration of negative differential mobility within the bulk.Consequently, a negative differential conductance is caused in thecurrent-voltage characteristics and leads to thermodynamic instability.Therefore, a domain is generated which transits from the cathode side tothe anode side. Repetition of this phenomenon results in vibratingcurrent (oscillation).

The oscillating frequency of a Gunn diode is determined by the distanceof transit of the domain. In case of Gunn diodes for millimeter waves,this distance of transit needs to be extremely short (1 to 2 μm). Inaddition, the product of an impurity concentration and a distance oftransit for the domain (active layer) needs to be set to be a specifiedvalue (e.g. 1×10¹²/cm²) to obtain sufficient oscillating efficiency,while the impurity concentration of the active layer becomes rather highin high frequency zones like those of millimeter waves since theoscillating frequency is non-ambiguously determined by the thickness ofthe active layer. The current concentration during operation isdetermined by the product of the impurity concentration of the activelayer and a saturation electron speed, and in zones of the millimeterwaves, the temperature of the active layer is increased owing to theincrease in current concentration, whereby the oscillating efficiency isdecreased.

In order to solve such problems, measures had been taken withconventional Gunn diodes for millimeter waves such as employing amesa-type arrangement to use elements including the active layer ofextremely small sizes, having diameters of approximately several tens ofμm, and assembling the diodes within pill-type packages comprised with aheat portion made of diamond or similar material of favorable thermalconductivity which greatly affects oscillating efficiency on which themost important performance indices are dependent.

A sectional view of gallium arsenide Gunn diode element 100 ofconventional mesa-type arrangement is shown in FIG. 29.

On to a semiconductor substrate 101 of high concentration n-type galliumarsenide, there are sequentially laminated, through MBE method, a firstcontact layer 102 of high concentration n-type gallium arsenide, anactive layer 103 of low concentration n-type gallium arsenide, and asecond contact layer 104 of high concentration n-type gallium arsenide,and it is employed a mesa-type arrangement in order to reduce thetransit space for the electrons.

Thereafter, a rear surface of the semiconductor substrate 101 islaminated, a cathode electrode 105 is formed onto this rear surface ofthe semiconductor substrate 101 while an anode electrode 106 is formedon the surface of the second contact layer 104, and by performingelement separation, the Gunn diode element is completed.

The Gunn diode element 100 thus obtained is built-in in a pill-typepackage 110 as shown in FIG. 30. This pill-type package 110 comprises aheat sink electrode 111 and a cylinder 112 of glass or ceramics thatserves as an enclosure for enclosing the Gunn diode element 100, whereinthe cylinder 112 is brazed by hard-soldering to the heat sink electrode111. The Gunn diode element 100 is electro-statically attracted by abonding tool of sapphire material or the like (not shown) and is adheredto the heat sink electrode 111.

Further, the Gunn diode element 100 and a metal layer provided at a tipof the cylinder 112 are connected by a gold ribbon 113 throughthermo-compression bonding or the like. After connecting the gold ribbon113, a lidlike metallic disk 114 is brazed onto the cylinder 112 tocomplete the building-in to the pill-type package 110.

An example of a structure for assembling the Gunn diode that has beenbuilt-in in the pill-type package 110 to a microstrip line 120 is shownin FIG. 31. One of the two electrodes 111, 114 of the pill-type package110 is pierced through a hole formed in a flat insulating substrate 121of e.g. alumina and is electrically connected to a ground electrode 122formed on a rear surface of the flat insulating substrate 121, while theother one is connected by a gold ribbon 123 to a signal line 124 formedon the plate substrate 121 as a microstrip line.

NRD guide circuits are paid attention to as transmission lines formicrowaves, especially of millimeter wave zones of not less than 30 GHz,since they present lower insertion losses than compared to microwavestrip lines, and since manufacturing of transmission line is easier thancompared to waveguides.

This NRD guide circuit is arranged in that a dielectric strip line, inwhich propagation of electromagnetic waves is performed, is pinchedbetween two parallel plates of conductive metal, Since the opposingdistance between the parallel plates is set to be not more than half ofthe free space wavelength of the used frequency, electromagnetic wavesare intercepted and its radiation is restricted at portions other thanthe dielectric strip line, electromagnetic waves can be propagated withlow losses along the dielectric strip line.

Oscillators arranged of such a NRD guide circuit and Gunn diodes of 35GHz and 60 GHz zone have been developed which are capable of producingoutput power which are equivalent to those of waveguides.

FIG. 32(a) is a view showing an arangement of a conventional NRD guideGunn oscillator. This is arranged in which a mount 320 is provided in aspace between parallel plates 201, 202, being mounted with a dielectricstrip line 203 as well as Gunn diode 310, High frequency outputoscillated by the Gunn diode 310 is derived to the dielectric strip line203 via a resonator 330. FIG. 32(b) is a view showing a representativeexample of such resonator 330 comprised with a copper layer portion 331patterned through etching a copper layer of a Teflon copper-cladlaminate. By adjusting the width or length of the copper layer portion331, the output frequency can be adjusted,

FIG. 33 is a view showing the arrangement of the mount 320. The Gunndiode 310 is set in a cylindrical portion 321, and bias voltage isapplied thereto via a bias choke 340 connected to aside the cylindricalportion 321. The bias choke 340 is obtained by patterning throughetching a Teflon copper-clad laminate and by hacking a portion of thelaminated plate of the cylindrical portion 321 such that a copper layerportion remains to be connected to a lid for connecting portion 341. Acathode electrode of the Gunn diode 310 is connected onto a heat sink322 of the mount 320. The heat releasing base 322 is insulated andseparated from the lid 341 by a cylindrical ceramic 342, and the lid 341is connected to an anode electrode of the Gunn diode 310 via a ribbon343.

Conventional Gunn diode elements 100 (FIG. 30) are formed throughchemical wet etching by employing a photoresist as an etching mask toobtain the above described mesa type arrangement. However, since etchingis progressed not only in the depth direction but also simultaneously inlateral directions, in this etching method, it is presented a drawbackduring manufacture that control of the transit space of the electrons(active layer) is made very difficult, whereby ununiformity inelectrical characteristics of Gunn diode element is caused.

It was also presented a drawback at the time of building-in the Gunndiode element in a pill-type package 110 that the bonding toolintercepted one's field of view during adhesion of the Gunn diodeelement 100 to the he at sink electrode 111 so that the heat releasingsink is 111 could not be directly viewed at. Consequently, theefficiency of building-in operation was quite poor.

Further utilization of a gold ribbon 123 (FIG. 31) for assemabling thepill-type package 110 incorporated with the Gunn diode element 100 tothe microstrip line 120 arranged on the plate substrate 121 resulted ingeneration of parasitic inductance, whereby ununiformity in electricalcharacteristics was caused during the assembly,

Manufacture of the above described NRD guide Gunn oscillator isdifficult since it employs a special mount 320, and the operatingefficiency was very poor since the substrate needed to be hacked toexpose the lid 341 of the bias choke 340.

Utilization of the ribbon 343 for connecting the anode electrode of theGunn diode 310 to the lid 341 resulted in generation of parasiticinductance, whereby variations in a characteristics were caused.

It is an object of the present invention to provide Gunn diodes,fabricating methods thereof and structures for assembling the same whichsolve the above described problems which are caused during, fabricatingbuilding-in and assembly.

It is another object of the present invention to provide a NRD guideGunn oscillator free of the above described problems.

SUMMARY OF THE INVENTION

For this purpose, the Gunn diode according to the first invention is aGunn diode which is formed by sequentially laminating a firstsemiconductor layer, an active layer and a second semiconductor layeronto a semiconductor substrate, comprising first and second electrodesarranged on the second semiconductor layer for impressing voltage on theactive layer, and a concave portion which is cut from around the firstelectrode in a direction of the second semiconductor layer and theactive layer and which subdivides the second semiconductor layer and theactive layer to which the first electrode is connected as a region whichfunctions as a Gunn diode.

The Gunn diode according to a second invention is so arranged that aconductive film is provided within the concave portion for shortingbetween the second electrode and the first semiconductor layer of thefirst invention.

The Gunn diode according to the third invention is so arranged that thefirst and second electrodes are formed of an underlying electrode layerand conductive protrusions successive to the underlying electrode layersuch that their upper surfaces assume a substantially identical levelheight.

The Gunn diode according to the fourth invention is so arranged that theconductive protrusion of the first electrode is formed substantially ina central portion and in that the conductive protrusions of the secondelectrode are formed at both sides thereof in the first to thirdinventions,

The Gunn diode according to the fifth invention is so arranged that anarea for the first electrode is set to be not more than {fraction(1/10)} of an area for the second electrode in the first to fourthinventions.

The Gunn diode according to the sixth invention is so arranged thatthere are provided at least two first electrodes and concave portionswhich have been cut from around the first electrodes in the first tofifth inventions.

The Gunn diode according to the seventh invention is so arranged thatthe semiconductor substrate, the first semiconductor layer, the activelayer and the second semiconductor layer are formed of compoundsemiconductors such as gallium arsenide or indium phosphide in the firstto sixth inventions

The Gunn diode according to the eighth invention is so arranged that thesecond semiconductor layer and the active layer being successive to thesecond electrode are substituted as a single semiconductor layer or aconductive layer in the first to seventh inventions.

The Gunn diode according to the ninth invention is so arranged that athird electrode is formed on a rear surface of the semiconductorsubstrate, in that the third electrode and first electrode are used forimpressing voltage onto the active layer, and in that the secondelectrode is made to be for the spacers in the first to eighthinventions.

The fabricating method for a Gunn diode according to the tenth inventionis so arranged that it comprises a first step of sequentially laminatingand forming a first semiconductor layer which serves as a first contactlayer, an active layer, and a second semiconductor layer which serves asa second contact layer onto a semiconductor substrate, a second step offorming first and second electrodes of specified shapes onto the secondcontact layer, and a third step of removing the second semiconductorlayer and the active layer through dry etching wherein the first andsecond electrodes are used as masks.

The fabricating method for a Gunn diode according to the eleventhinvention is so arranged that the second step includes a step offorming, after forming an underlying electrode layer for the first andsecond electrodes of specified shapes, conductive protrusions on theunderlying electrode layer such that their heights are substantiallyidentical with each other in the tenth invention,

The fabricating method for a Gunn diode according to the twelfthinvention is so arranged that the semiconductor substrate, the firstsemiconductor layer, the active layer and the second semiconductor layerare formed of compound semiconductors such as gallium arsenide or indiumphosphide in the tenth or eleventh inventions.

The structure for assembly of the Gunn diode of the thirteenth inventionis so arranged that a surface ground electrode is formed on a surface ofa microstrip substrate obtained by forming a signal electrode on asurface of a semi-insulating plate substrate and a ground electrode on arear surface thereof, wherein the surface ground electrode is connectedto the ground electrode on the rear surface through a via hole, and thatthe first and second electrodes of the Gunn diode of the first to eighthinventions are respectively connected and mounted to the signalelectrode and the surface ground electrode.

The structure for assembly of the Gunn diode of the fourteenth inventionis so arranged that the first and second electrodes of the Gunn diode ofthe first to eighth inventions are respectively connected and mounted toa signal electrode and a pair of ground electrodes of a coplanarwaveguide obtained by forming the signal electrode and the pair ofground electrodes on a surface of a semi-insulating plate substrate.

The structure for assembly of the Gunn diode of the fifteenth inventionis so arranged that one end of the signal electrode is open at length Lfrom a portion to which the first electrode of the Gunn diode isconnected, wherein a first electrode portion of the length L acts as aresonator and wherein an oscillating frequency is determined by thelength L.

The structure for assembly of the Gunn diode of the sixteenth inventionis so arranged that fourth and fifth electrodes are formed at a heatsink made of an insulating substrate, wherein the first electrode of theGunn diode of the ninth invention is directly connected and mounted tothe fourth electrode of the heat sink and the second electrode of theGunn diode to the fifth electrode of the heat sink.

The structure for assembly of the Gunn diode of the seventeenthinvention is so arranged that a hole is formed on a microstrip substrateobtained by forming a signal electrode on a surface of a semi-insulatingplate substrate and a ground electrode which concurrently acts as a heatsink on a rear surface thereof, the hole extending from the surface tothe ground electrode on the rear surface, wherein the fifth electrode ofthe heat sink of the sixteenth invention is connected to the groundelectrode and wherein the third electrode of the Gunn diode of thesixteenth invention is connected to the signal electrode of themicrostrip line through a conductive line within the hole

The structure for assembly of the Gunn diode of the eighteenth inventionis so arranged that an oscillating circuit, which oscillates at aspecified frequency, is arranged of the signal electrode, the groundelectrode and the Gunn diode, or by further adding a dielectricresonator thereto, in the thirteenth to seventeenth inventions.

The structure for assembly of the Gunn diode of the nineteenth inventionis so arranged that a portion of the signal electrode that functions asan electrode of the oscillating circuit is at least partially covered bya plate substrate of conductive material, and in that the conductiveportion of the plate substrate is connected to the ground electrode inthe eighteenth invention.

The structure for assembly of the Gunn diode of the twentieth inventionis so arranged that a resistivity of the plate substrate of themicorstrip line or coplanar waveguide is not less than 10⁶ Ωcm, and athermal conductivity is not less than 140 W/mK in the thirteenth tonineteenth inventions.

The structure for assembly of the Gunn diode of the twenty-firstinvention is so arranged that the plate substrate of the microstrip lineor the coplanar waveguide is made of at least one of AlN, Si, SiC ordiamond in the thirteenth to twentieth inventions.

The NRD guide Gunn oscillator of the twenty-second invention is obtainedby disposing two parallel plates of metal at a distance that is not morethan half a free space wavelength of an used frequency aid combining aNRD guide circuit pinching and holding a dielectric strip line betweenthe parallel plates and a Gunn diode, wherein the NRD guide Gunnoscillator comprises a plate substrate of insulating or semi-insulatingmaterial on which surface there are formed a signal electrode connectedto a signal line and a ground electrode insulated with respect to thesignal electrode, a Gunn diode being formed with an anode electrode anda cathode electrode on a same plane wherein one of the electrodes isconnected to the signal electrode of the plate substrate and the otherone is connected to the ground electrode, and a heat sink for supportinga rear surface of the plate substrate with respect to the other parallelplate, wherein a tip of the signal line of the plate substrate iselectromagnetically combined to the dielectric strip line.

In the twenty-third invention, the plate substrate to which the Gunndiode is connected and mounted is parallel with respect to the parallelplate, and the signal line is electromagnetically combined thereto in avertical direction with respect to the dielectric strip line in thetwenty-second invention.

In the twenty-fourth invention, the plate substrate to which the Gunndiode is connected and mounted is parallel with respect to the parallelplate, a progressing direction of electromagnetic waves of the signalline is identical with a progressing direction of electromagnetic wavesof the dielectric strip line, and the signal line is electromagneticallycombined to a base end portion of the dielectric strip line in thetwenty-second invention.

In the twenty-fifth invention, a posture of the parallel substrate towhich the Gunn diode is connected and mounted is changed from a parallelone to a vertical one with respect to the parallel plate in thetwenty-third or twenty-fourth invention.

In the twenty-sixth invention, the signal line is a suspended microstripline, microstrip waveguide or coplanar line in the twenty-second totwenty-fifth inventions.

In the twenty-seventh invention, the parallel substrate comprises anelectrode for grounding on a rear surface thereof, and the electrode forgrounding is connected to the ground electrode through a via hole in thetwenty-second to twenty-sixth invention,

The NRD guide Gunn oscillator of the twenty-eighth invention is obtainedby disposing two parallel plates of metal at a distance that is not morethan half a free space wavelength of an used frequency and combining aNRD guide circuit pinching and holding a dielectric strip line betweenthe parallel plates and a Gunn diode, wherein the NRD guide Gunnoscillator comprises a plate substrate of insulating or semi-insulatingmaterial on which surface there are formed two signal electrodesconnected to both ends of a signal line and a ground electrode insulatedwith respect to the respective signal electrodes, two Gunn diodes beingrespectively formed with an anode electrode and a cathode electrode on asame plane wherein one of the electrodes is connected to the signalelectrodes of the plate substrate and the other one is connected to theground electrode, and a heat sink for supplying a rear surface of theplate substrate with respect to the other parallel plate, wherein asubstantially central portion of the signal line of the plate substrateis electromagnetically combined to the dielectric strip line.

In the twenty-ninth invention, a length of the signal line is set to besubstantially half of a guide wave length of the signal line or aninteger multiple thereof in the twenty-eighth invention

In the thirtieth invention, the plate substrate to which the Gunn diodesare connected and mounted is vertical with respect to the parallelplate, and the substantially central portion of the signal line iselectromagnetically combined with an end portion of the dielectric strip(line) in the twenty-eighth or twenty-ninth inventions.

In the thirty-first invention, a posture of the plate substrate to whichthe Gunn diodes are connected and mounted is changed from a vertical oneto a parallel one with respect to the parallel plate in the thirtiethinvention.

In the thirty-second invention, the signal line is a suspendedmicrostrip line, microstrip line or coplanar line in the twenty-eighthto thirty-first inventions.

In the thirty-third invention, the plate substrate comprises anelectrode for grounding on a rear surface thereof, and the electrode forgrounding is connected to the ground electrode through a via hole in thetwenty-eighth to thirty-second inventions.

BRIEF EXPLANATION OF THE DRAWINGS

FIGS. 1(a) and 1(b) are views showing a Gunn diode element according toa first embodiment of the present invention, wherein FIG. 1(a) is a topview and FIG. 1(b) a sectional view,

FIG. 2 is a view for explaining fabrication steps of the above Gunndiode element;

FIG. 3(a) and FIG. 3(b) are sectional views showing an alternativeexample of the above Gunn diode element;

FIG. 4 is a cross view of a second embodiment in which the above Gunndiode element is assembled in a microstrip substrate;

FIG. 5 is a cross view of an alternative example of the structure forassembly of FIG. 4;

FIG. 6(a) and FIG. 6(b) are top views of the structure for assembly ofthe Gunn diode element;

FIG. 7 shows variation of oscillation frequencies and RF power as afunction of length L of electrode 32B in case the Gunn diode element isassembled as an oscillator;

FIG. 8 shows spectrum of the oscillation in case the Gunn diode elementis assembled in direction (a) in FIG. 6;

FIG. 9 shows spectrum of the oscillation in case the Gunn diode elementis implemented in direction (b) in FIG. 6;

FIG. 10 is a cross view in which a plate substrate is additionallyassembled to the structure for assembly of FIG. 5;

FIG. 11 is a cross view of a third embodiment in which the above Gunndiode element is implemented in a coplanar waveguide;

FIG. 12 is a cross view of an alternative example of the structure forassembly of FIG. 11;

FIG. 13 is a cross view in which a plate substrate is additionallyassembled to the structure for assembly of FIG. 12;

FIGS. 14(a) and 14(b) show a fourth embodiment in which the Gunn diodeis implemented in a heat sink in a facing-down posture, wherein FIG.14(a) is a top view of the heat sink and FIG. 14(b) a sectional view ofthe assembled condition;

FIG. 15 is a sectional view showing a condition in which the Gunn diodeelement assembled in the heat sink as shown in FIG. 14 is furtherassembled in a microstrip line;

FIGS. 16(a) and 16(b) are views showing a Gunn diode element of a fifthembodiment of the present invention, wherein FIG. 16(a) is a top viewand FIG. 16(b) a sectional view;

FIG. 17 is a diagram showing characteristics of output power andconversion efficiency corresponding to a number of mesa-type structuredportions of a Gunn diode element of a specified sum of area;

FIG. 18 is a diagram showing characteristics of output power andconversion efficiency corresponding to a number of mesa-type structuredportions of a Gunn diode element of another specified sum of area;

FIG. 19 is an explanatory view of an assembling condition for a Gunndiode employed for measuring characteristics in FIG. 17 and FIG. 18;

FIG. 20(a) is a cross view of a NRD guide Gunn oscillator according toan embodiment of the present invention, and

FIG. 20(b) a side view thereof;

FIG. 21(a) is a plan view of a line substrate, and

FIG. 21(b) a view of a rear surface thereof;

FIG. 22(a) is a top view of a Gunn diode,

FIG. 22(b) a sectional view thereof, and (c) a sectional view of a Gunndiode according to an alternative example;

FIG. 23 is a cross view of a NRD guide Gunn oscillator according toanother embodiment of the present invention;

FIG. 24 is a cross view of a NRD guide Gunn oscillator according tostill another embodiment of the present invention;

FIG. 25 is a cross view of a NRD guide Gunn oscillator according to yetanother embodiment of the present invention;

FIG. 26(a) is a top view of a line substrate, and

FIG. 26(b) a view of a rear surface thereof;

FIG. 27(a) is a top view of a Gunn diode,

FIG. 27(b) a sectional view thereof, and FIG. 27(c) a sectional view ofa Gunn diode of an alternative example;

FIG. 28 is a cross view of a NRD guide Gunn oscillator according toanother embodiment of the present invention;

FIG. 29 is a sectional view of a conventional Gunn diode of mesa-typestructure;

FIG. 30 is a sectional view of a conventional Gunn diode of mesa-typestructure that is incorporated in pill-type package;

FIG. 31 is an explanatory view in which the pill-type package is mountedon a microstrip line:

FIG. 32(a) is a cross view of a conventional NRD guide Gunn oscillator,and

FIG. 32(b) a cross view of a resonator;

FIG. 33(a) is a cross view of a mount of the NRD guide Gunn oscillatoras shown in FIG. 32 and FIG. 33(b) a sectional view taken along the lineB—B.

DETAILED DESCRIPTION Embodiment 1

FIGS. 1(a) and 1(b) are diagrams showing a structure of a Gunn diodeelement 10 of galium arsenide according to a first embodiment of thepresent invention, wherein FIG. 1(a) is a top view and FIG. 1 (b) is asectional view, FIG. 2 is a view showing fabrication steps.

Fabrication steps will now be explained along contents of FIG. 2. Onto asemiconductor substrate 11 of n-type gallium arsenide having an impurityconcentration of 1 to 2×10¹⁸ atom/cm³, there are sequentially laminatedthrough MBE method a first contact layer 12 of n-type gallium arsenidehaving an impurity concentration of 2×10¹⁸ atom/cm³ and a thickness of1.5 μm, an active layer 13 of n-type gallium arsenide having an impurityconcentration of 1.2×10¹⁶ atom/cm³ and a thickness of 1.6 μm and asecond contact layer 14 of n-type gallium arsenide having an impurityconcentration of 1×10¹⁸ atom/cm³ and a thickness of 0.3 μm to obtain asemiconductor substrate with laminated lagers.

Onto the second contact layer 14, there is patterned a photoresist thatis opened at regions on which a cathode electrode and an anode electrodeare to be formed, and a metal film (underlying electrode layer)of AuGe,Ni and Au that is in ohmic contact with the second contact layer 14 isvapor-deposite a thereon. After removing the photoresist, a heattreatment (sintering) is performed, and cathode electrode 15 and anodeelectrode 16 are formed on the second contact layer 14 in a separatemanner (FIG. 2(a)). As shown in FIG, 1, the planar shape of the cathodeelectrode 15 is oblong and the planar shape of the anode electrode 16 isround, while these might alternatively be oval or substantially square.

Next, photoresist 17 is patterned as to leave a part of the surfaces ofthe cathode electrode 15 and anode electrode 16 open, and bumps(electrodes) 18, 19 which are conductive protrusions of Au or the likeare formed in the open portions through precipitation by electrolyticplating or non-electrolytic plating (FIG. 2(b)).

Next, after exposing the second contact layer 14 formed with the cathodeelectrode 15 and anode electrode 16 through removing the photoresist 17,the cathode electrode 15 and anode electrode 16 are used as masks whenremoving the second contact layer 14 and active layer 13 through dryetching such as reactive ion etching (RIE) employing chlorine gas or thelike to form a substantially mesa-type or vertical concave portion 20around the anode electrode 16 FIG. 2(c)). In this manner, a targetedconcave portion 20 can be accurately formed through etching in avertical direction through self-alignment using the cathode electrode 15and the anode electrode 16 as masks.

The area of the active layer 13 to which the anode electrode 16 whichhas been sub-divided by the concave portion 20 is connected is set to bean area (transverse cross section) with which a specified operatingcurrent of the Gunn diode can be obtained. That is, this area is set tobe an area that can function as a Gunn diode. Further, an area of theactive layer 13 to which the cathode electrode 15 is connected is set tobe not less than ten times as large as an area of the active layer 13 towhich the anode electrode 16 is connected, and the electric resistanceof a semiconductor laminated portion below the cathode electrode 15 ismade to be not more than {fraction (1/10)} of the electric resistance ofa semiconductor laminated portion below the anode electrode 16. Withthese arrangements, this portion is not made to function as a Gunn diodebut to function as a resistance of substantially low value, and thecathode electrode 15 is substantially connected to the first contactlayer 12. The area ratio of the active layer 13 needs to be not lessthan 10, and preferably not less than 100, since a ratio of below 10would not be effective but only results in a decreased conversionefficiency.

It should be noted that while the cutting depth of the concave portion20 is set to be a depth that is obtained by totally removing the activelayer 13, but it might alternatively be arranged that a part of theactive layer 13 remains or that the cutting reaches to some extent intothe first contact layer 12.

It should be noted that while the area of the active layer below thecathode electrode has been set to be larger than that of the anodeelectrode, it might be employed an reversed arrangement in which thearea of the active layer below the anode electrode is larger than thatof the cathode electrode. That is, the anode electrode and cathodeelectrode are mutually interchangeable. While elimination ofconcentration gradients in the impurity concentration of the activelayer 13 enables the interchange between the anode 19 and cathode 18, inthe presence of concentration gradients, the electrode of lowerconcentration is set to be the cathode electrode and the electrode ofhigher concentration is set to be the anode electrode.

Next, the rear surface of the semiconductor substrate 11 is ground forlamination in accordance with the ordinary fabricating processes forGunn diodes such that the thickness of the whole Gunn diode becomesapproximately 60 μm. Thereafter, if required, a metal film 21 of AuGe,Ni, Au, Ti, Pt or Au that is in ohmic contact with the semiconductorsubstrate 11 is vapor-deposited onto the rear surface of thesemiconductor substrate 11, and a heat treatment is performed (FIG.2(d)).

While the metal film 21 that is formed on the rear surface of thesemiconductor substrate 11 is not necessarily required, it mightfunction as a cathode electrode substituting the cathode electrode 15 inthe case of an structure for assembly (FIG. 15) as will be describedlater is taken. In such a case, there will be no restrictions to set thearea ratio between the caathode electrode 15 and the anode electrode 16to be not more than {fraction (1/10)} as it has been described above.

As explained so far, the Gunn diode 10 according to the presentembodiment is so arranged that it comprises, in a separate manner, aportion which functions as a Gunn diode and a low resistance layerportion which functions as a voltage impressing path from the exteriorto the first contact layer 12 of the Gunn diode portion through theprovision of the concave portion 20 in the semiconductor laminatedportion to surround the anode electrode 16. With this arrangement, boththe cathode electrode 15 and anode electrode 16 can be formed on theupper surface of the second contact layer 14. In other words, thecathode electrode 15 and the anode electrode 16 can be arranged on asame surface, whereby great advantages can be obtained in terms ofassembly or heat dissipation as will be described later.

Since etching for defining a region for determining an operationalcurrent (a portion that functions as a Gunn diode) is performed throughdry etching in a self-alignment method by utilizing electrodes formedabove the region as masks, variations in manufacture can be decreasedthan compared to conventional chemical wet etching, and the yield(factor of good products) can be made high.

FIG. 3(a) is a view showing an alternative example 10′ of the Gunn diodeelement 10 shown in FIG. 1(b), which is so arranged that a conductivefilm 22 is provided within the concave portion 20 and that the firstcontact layer 12 and the cathode electrode 15 are shorted. With thisarrangement, influences of parasitic resistance can be prevented in casethe parasitic resistance between the cathode electrode 15 and the firstcontact layer 12 is large, and voltage impressed on the cathodeelectrode 15 can be transmitted to the first contact layer 12 withhardly no losses.

By further developing the idea of this Gunn diode element 10′, a Gunndiode element 10″ as shown in FIG. 3(b) can be provided wherein thecathode electrode 15 is directly formed on the upper surface of thefirst contact layer 12, bumps 18 are formed on the surface thereof, andthe remaining arrangements are identical with those as shown in FIG.1(b), whereby the upper surfaces of the bumps 18, 19 are aligned atidentical height levels. In the Gunn diodes 10′, 10″, there will be norestrictions to set the area ratio between the cathode electrode 15 andthe anode electrode 16 to be not more than {fraction (1/10)} as it hasbeen described above.

Embodiment 2

FIG. 4 is a view showing one example of awn arrangement in which a Gunndiode element 10 is assembled on a planar circuit substrate forming amicrostrip line 30 to form an oscillator. A signal electrode 32 isformed on a plate substrate 31 of semi-insulating material such as AlN(aluminum nitride), Si (silicone), SiC (silicone carbide) or diamondhaving a favorable resistivity of not less than 10⁵ Ωcm and a thermalconductivity of not less than 140 W/mK, and the rear surface thereof isformed a ground electrode 33. 34 denotes via holes filled with tungsten,that connect the ground electrode 33 on the rear surface and surfaceground electrode 35 formed on the upper surface.

A bump 19 of an anode electrode of the Gunn diode element 10 isconnected to the signal electrode 32, and bumps 18 of cathode electrodesare connected to the ground electrode 35. 32A denotes an electrode of abias portion for supplying power source voltage to the Gunn diodeelement 10, 32B an electrode for making up an resonator formed by themicrostrip line including the Gunn diode element 10, 36 a condenserportion for performing direct-current cut, and 32C an electrode of asignal output portion formed by the microstrip line.

In this structure for assembly, the Gunn diode element 10 is set in afacing-down posture and the bumps 18, 19 are directly connected to theelectrodes 35, 32 without employing a gold ribbon. With thisarrangement, generation of parasitic inductance owing to connectionthrough the gold ribbon can be eliminated, and an oscillator with hardlyno variations in characteristics can be realized.

Since heat generated in the Gunn diode element 10 is dissipated, via thebumps 18, 19, to the substrate 31 which also functions as a heat sink,heat dissipating effects can be improved. Further, since the bumps 18 ofthe cathode electrode are positioned on both sides of the bump 19 insuch an assembly of the Gunn diode element 10, it can be prevented thatexcessive mechanical load is applied to the anode electrode.

In FIG. 5, the electrode 32A of the bias portion is provided on the sideof the electrode 32C of the signal output portion of the oscillator asshown in FIG. 4. The plane of the plate substrate 31 arranged as in FIG.5 would look like FIG. 6(a), and by adjusting length L of the electrodes32B which is open at its tip, the oscillation frequency and output powercan be set. FIG. 7 shows the oscillating characteristics of the circuitshown in FIGS. 6(a), (b), wherein the characteristic impedance of theelectrode 32C is set to 50Ω, and the characteristic impedance of theelectrode 32B to 35Ω.

FIG. 8 shows the oscillating spectrum at a center frequency of 58.68GHz, and it can be seen that the phase noise is −85 dBc/Hz at 100 KHzoff carrier, this value being more favorable than compared to those ofGunn diode oscillators employing a waveguide cavity. While the value is−46.7 dBc in FIG. 8, it becomes −85 dBc/Hz from the following equation

 −47.6 dB+2.5 dB−10 log (1 Hz/(10 Hz×1.2))=−85 dB

It should be noted that in case the oscillator is arranged as shown inFIG. 6(b) wherein the bump 19 of the anode electrode in the center ofthe Gunn diode element 10 is connected to surface ground electrodes 35′that are connected to the ground electrode on the rear surface throughthe via, holes, and one of the bumps 18 of the cathode electrodes onboth sides is connected to electrode 32B′ of the resonator and the otherone to the electrodes 32C for output, the phase noise is −75 dBc/Hz at100 KHz off carrier with a center frequency of 61.63 GHz as shown inFIG. 9 (while it should be again noted this value is obtained from theabove equation based on the value −36.7 dBc/Hz in FIG. 9). It can thusbe understood that this arrangement is inferior to the connectingstructure as shown in FIG. 6(a) by 10 dB.

This is considered to be due to the fact that the semiconductorsubstrate 11 of the Gunn diode element 10 is grounded via the bumps 18or surface ground electrode 35 in the structure or connection of FIG.6(a), and this semiconductor substrate 11 functioning as a shieldingplate, decrease in Q owing to radiation loss of the oscillator can berestricted whereby the phase noise is improved.

In FIG. 10, additional surface ground electrodes 35′ are formed alongboth sides of the electrodes 32B to be in alignment with surface groundelectrodes 35 in the oscillator as show in FIG. 5, wherein theadditional electrodes are connected to the ground electrode 33 on therear surface through via holes (not shown) and wherein a conductiveplate substrate 80 has been provided to cover the electrode 32B makingup the oscillator. This plate substrate 80 comprises bumps 81 forconnection with the surface ground electrodes 35′.

In the arrangement as shown in FIG. 10, the conductive plate substrate80 is grounded via the bumps 81 and the surface ground electrodes 35′,whereby the radiation loss in the resonator can be further restricted torealize a resonator of high Q. The substrate of the plate substrate 80itself maybe of semi-insulating material as long as at least a partthereof is covered by metallic electrodes. A similarly high Q can beachieved by substituting the plate substrate 80 by a Gunn diode element10 having a larger chip size and by covering the electrodes 32B by thesemiconductor substrate 11 of the Gunn diode element 10. The surfaceground electrodes 35′ may be formed by extending the surface groundelectrodes 35.

Embodiment 3

FIG. 11 is a view showing an example of an arrangement in which a Gunndiode element 10 is implemented in a circuit substrate making up acoplanar waveguide 40. 41 denotes a semi-insulating plate substrate madeof the same material as the above described substrate 31, on whichsurface there are formed a signal electrode 42 forming a signal line anda pair of grounding electrodes 43 as to pinch the same between.

Here, bump 19 of the anode electrode of the Gunn diode element 10 isdirectly connected to the signal electrode 42 in the center, and bumps18 of the cathode electrodes are directly connected to ground electrodes43 on both sides. With this arrangement, applied voltage between thesignal electrode 42 and ground electrodes 43 is applied between theanode electrode and cathode electrodes of the Gunn diode element 10,whereby oscillation can be generated. This structure for assembly asshown in FIG. 11 presents functions and effects similar to thestructures for assembly as shown in FIG. 4, FIG. 5 and FIG. 10 such asstabilizing characteristics, improving heat dissipating effects, orprotecting the anode electrode.

In FIG. 12, electrode 42A functioning as a bias portion for applying +30V is formed in succession to signal line 42. A choke is formed by theground electrode 43 as to surround the electrode 42A in order to easeinfluences to the power source. The oscillation frequency and outputpower can similarly be set by adjusting the length from a portion of theGunn diode element 10 of the electrode 42B making up the oscillator upto its open tip. 42C denotes an electrode-of a signal output portion.

FIG. 13 is based on the same idea as that of the above described FIG.10, wherein an upper surface of the electrode 42B making up theoscillator is covered by conductive plate substrate 80, and bumps 81 onboth sides of the plate substrate 80 are connected to grounding electricconductors 43. This arrangement makes it possible to restrict radiationlosses in the resonator and to realize a resonator having a high Q.

Embodiment 4

FIG. 14 is a view showing a heat dissipating structure of Gunn diodeelement 10. 50 denotes a heat sink employing a diamond substrate 51 onwhich there are formed electrodes 52 to which bumps 18 of cathodeelectrodes of the Gunn diode element 10 are connected, and electrode 53to which a bump 19 of anode cathode 19 is connected, Electrodes 52 areseparated from the electrode 53 in an independent manner and theelectrodes 53 are connected to the ground electrode 54.

While heat is generated at the semiconductor laminated portion of theGunn diode element 10 that corresponds to the anode electrodefunctioning as the Gunn diode, this heat is transferred to the heat sink50 via the bumps 18, 19 (mainly bumps 19) for performing cooling.

FIG. 15 is a view showing the structure for assembly of the Gunn diodeelement 10 of FIG. 14 assembled into microstrip line 60. The heat sink50 implemented with the Gunn diode element 10 is adhered, within hole 61formed in the microstrip 60, to a ground electrode 62 which concurrentlyserves as a heat dissipating base, and a signal electrode 64 on theplate substrate 63 made of alumina and a cathode electrode 21 on therear surface of the Gunn diode element 10 are connected through goldribbon 28.

In this arrangement, applied voltage between the signal electrodes 64and the ground electrodes 62 is applied between the cathode electrode 21and anode electrode 16 through the gold ribbon 28 and electrodes 53, 54of the heat sink 50. At this time, the bumps 18 of the cathodeelectrodes 15 function as spacers for maintaining the facing-downposture from both sides and do not function as a transmitting path forcurrent. This arrangement is quite simple and enables a remarkabledecrease in costs than compared to arrangements employing conventionalpill-type packages 110.

Embodiment 5

FIG. 16 is a view showing an arrangement of an alternative example of aGunn diode element 10A, wherein (a) is a top view and (b) a sectionalview. In this arrangement, four anode electrodes 16 are individuallyformed and four concave portions 20 corresponding thereto make up fourGunn diode portions of mesa-type structure. Since voltage is commonlyapplied to the individual Gunn diode portions of mesa-type structure,these are in parallel connected conditions during operation.

With this arrangement, the radius of the mesa-type structured portionscan be made small and since the heat dissipating effect is remarkablyhigher than compared to a single Gunn diode portion of mesa-typestructure which area is identical to a sum of areas of four Gunn diodeportions of mesa-type structure, the conversion efficiency (ratio ofinput power to output power) or the output power is enabled to beremarkably high. It should be noted that the smaller the area of themesa-type structured portion becomes, the weaker its strength becomes sothat there may be a danger of destruction during the assembling stage.However, since the bumps 18 of the cathode electrodes are formed tosurround them and which substantially receive the load, there isactually no danger of destruction. It should also be noted that thenumber of individual Gunn diode portions of mesa-tpe arrangement is notlimited to four. The cross section of the plurality of Gunn diodes donot necessary be identical, neither are their sectional shapes (shapesof anode electrodes) limited to round shapes but may assume anyarbitrary shape.

FIG. 17 shows variation of conversion efficiency η (%) and output powerP (mW) as a function of the number of Gunn diode portions of mesa-typestructure. It can be understood that both the conversion efficiency aswell as the output power are increased in case the number of Gunn diodeportions of mesa-type structure are increased from four to nine withoutchanging the sum of areas of the anode electrodes. FIG. 18 shows similarvariation of conversion efficiency and output power in case the numberof Gunn diode portions of mesa-type structure is changed from four tosix with a different sum of areas of the anode electrodes, and it can beobserved a similar tendency.

It should be noted that such measurements have been performed under acondition in which they were assembled in a waveguide as shown in FIG.,19. 70 denotes a waveguide, 71 a conductive basement (anode) provided inthe waveguide 70, and 72 a solder for adhering an insulating substrate73 onto the basement 71. In the Gunn diode element 10A having aplurality of anode electrodes, bumps 18 of the cathode electrodes aresupported in a facing-down posture on the insulating substrate 73 viaelectrodes 74, and bump 19 of the anode electrode is connected to thebasement 71 through electrode 75, via hole 76 formed in the insulatingsubstrate 73 and the solder 72. A bias post 77 to which bias voltage isapplied is inserted into the waveguide 70, and a lower end thereof isconnected to electrode 21 on the rear surface of the Gunn diode element10A through gold ribbon 78.

It should be noted that while the above explanations are based on anexample in which gallium arsenide has been employed as semiconductors,similar effects can be achieved in case other compound semiconductorssuch as indium phosphide are used. Further, in case of arranging anoscillator by assembling the Gunn diode element to the above describedmicrostrip line or coplanar waveguide, it is also possible toadditionally provide a dielectric resonator.

Embodiment 6

FIG. 20 is a view showing an arrangement of a NRD guide Gunn oscillatoraccording to the sixth embodiment of the present invention. The NRDguide circuit is arranged in that a dielectric strip line 203 is pinchedbetween two metallic parallel-plates 201, 202 and is thus ofconventional arrangement. In the present embodiment, a Gunn diode 220 ismounted on an upper surface of a line substrate 210 that is supportedwith respect to the flat plate 202 via heat sink 230 arranged forgrounding, heat dissipating and height adjustment purposes,

The line substrate 210 is arranged, as shown in FIGS. 21(a) and (b), byforming onto an upper surface of a semi-insulating or insulating platesubstrate 211 which may, for instance, be of AlN, Si, SiC or diamondhaving a resistivity of not less than 10⁶ Ωcm and a thermal conductivityof not less than 140 W/mK), a signal line 212, a choke portion 213 forapplying direct-current bias onto the signal line 212, a signalelectrode 214 elongated to an end portion of the signal line 212, and apair of surface ground electrodes 215 disposed as to pinch the signalelectrodes 214 between. A ground electrode 216 is formed on a rearsurface of the substrate, and the surface ground electrodes 215 areconnected to the ground electrode 216 through via holes 217. The linesubstrate 210 does not comprise ground electrodes on the rear surface ofthe signal line 212 and thus forms a suspended microstrip.

The Gunn diode 220 is formed, as shown in FIGS. 22(a) and (b), bylaminating onto an upper surface of a semiconductor substrate 221 afirst contact layer 222, an active layer 223, a second contact layer 224and a metal layer 225, wherein a circular concave portion 226 is formedsuch that it substantially reaches from the metal layer 225 to the firstcontact layer 22. With this arrangement, the metal layer 225 issub-divided into anode electrode 225A and cathode electrode 225K, and adump 227 of Au that is easy to be bonded through thermo-compression isformed on the anode electrode 225A and a bump 228 similarly of Au on thecathode electrode 225K, such that their heights are respectively of samelevel. These bumps 227, 228 are equivalent to anode electrode 225A andcathode electrodes 225K respectively.

In one example, the semiconductor substrate 221 may be of n-type galliumarsenide having an impurity concentration of 1 to 2×10¹⁸ atom/cm³, afirst contact layer 222 of n-type gallium arsenide having an impurityconcentration of 2×10¹⁸ atom/cm³ and a thickness of 1.5 μm, an activelayer 223 of n-type gallium arsenide having an impurity concentration of1.2×10¹⁶ atom/cm³ and a-thickness of 1,6 μm, and a second contact layer224 of n-type gallium arsenide having an impurity concentration of1×10¹⁸ atom/cm³ and a thickness of 0.3 μm. It is also possible to employan alternative compound semiconductor such as indium phosphide insteadof gallium arsenide.

In the Gunn diode 220, an area of the sub-divided portion of the activelayer corresponding to the anode electrode 225A is set to be an areawith which a specified operating current of the Gunn diode can beobtained (transverse cross section. Further, an area of the active layercorresponding to the cathode electrode 225K is set to be not less thanten times as large as the area of the active layer corresponding to theanode electrode 225A, and the electric resistance of a semiconductorlaminated portion below the cathode electrode 225K is made to be notmore than {fraction (1/10)} of that of a semiconductor laminated portionbelow the anode electrode 223K. With these arrangements, this portion isnot made to function as a Gunn diode but to function as a resistance ofsubstantially low value.

It should be noted that the Gunn diode 220 may be alternativelyarranged, as shown in FIG. 22(c), to be Gunn diode 220′ in which thesecond contact layer 224 and active layer 223 underlying the cathodeelectrode 225K of FIG. 22(b) are omitted, wherein the cathode electrode225K is directly adhered to the first contact layer 222 and bumps 228thereof are provided to be of the same height levels as the bump 227 ofanode cathode 225A.

Assembly and mounting of the Gunn diode 220 to the plate substrate 211of the line substrate 210 is performed in that the bump 227 of the anodeelectrode 225A is connected to the signal electrode 214 and the pair ofbumps 228 of the cathode electrodes 225K to the pair of surface groundelectrodes 215 through thermo-compression bonding. By making the portionof the ground electrodes 216 of the line substrate 210 be connected tothe heat sink 230, they are grounded to the flat plate 202 through thisheat sink 230.

Assembly of the line substrate 210 to the NRD guide circuit isperformed, as shown in FIGS. 20(a) and (b), in that the plate substrate211 of the line substrate 210 is made parallel with respect to parallelplates 2011 202, and in that the tip of the signal line 212 approachesthe base portion of the dielectric strip line 203 from a verticaldirection.

In case direct-current voltage is applied on the choke portion 213,current is supplied through the signal line 212, signal electrode 214;Gunn diode 220; surface ground electrodes 215, via holes 217, groundelectrode on the rear surface 216, heat sink 230, and plate 202 in thisorder, whereby electromagnetic waves (microwaves) are generated at theGunn diode 220 and reach lateral surfaces of the dielectric strip line203 through the signal line 212. The electromagnetic waves are heretransformed into the NRD guide circuit (LSM mode) and are transmitted inthe dielectric strip line 203.

Since the choke portion 213 is formed on the plate substrate 211 in thisembodiment, the choke portion can be formed simultaneously with thesignal line 212, signal electrode 214, and surface ground electrodes 215through etching, whereby simple fabrication is enabled without the needof removal of the substrate to improve efficiency of assembling.Further, since the Gunn diode 220 is directly mounted onto the platesubstrate 211 in a face down posture, no parasitic inductance isgenerated unlike cases in which ribbons are used. Since heat generatedat the Gunn diode is transmitted to the heat sink 230 through bumps 227,228 or the plate substrate 211 that presents high thermal conductivity,the heat releasing effect can be improved. Also, since the Gunn diode220 is supported by the bumps 228 of the cathode electrodes 225K fromboth sides, it can be prevented that excessive load is applied to thesemiconductor laminated portion in the center that substantiallyfunctions as the Gunn diode.

It should be noted that while the portion of the signal line 212 and theportion to which the Gunn diode 220 is mounted are provided on a commonplate substrate 211, they may be arranged on different substrates andmay be connected by a conductive line such as gold ribbon. Also, the viaholes 217 may be replaced by ribbons or the like for connecting thesurface ground electrodes 215 to the ground electrode 216 on the rearsurface.

Further, while the signal line 212 of the line substrate 210 issuspended microstrip line in the above described example, the groundelectrode 216 may be provided on the whole rear surface of the platesubstrate 211 to make up a microstrip line. Such a line mayalternatively be a coplanar waveguide in which a signal line is providedin the center of the upper surface of the plate substrate 211 and a pairof ground electrodes are provided on the same plane as to pinch thesignal line between. In this case, the bump 227 of the anode electrode225A shall be connected to the signal line in the center and the bumps228 of the cathode electrodes 225K on both sides to the groundelectrodes.

Further, the anode electrode 225A and cathode electrode 225K of the Gunndiode 220 may be reversed, depending on the concentration gradient ofthe active layer, and in such a case, the polarity of voltage applied tothe choke portion 213 shall be suitably selected

FIG. 23 is a view showing an alternative example of a NRD guide Gunnoscillator. In this example, the direction of a line substrate 210 onwhich the Gunn diode is fabricated and supported by heat sink 230 is setto be parallel to parallel plates 201, 202 and such that a tip of itssignal line 212 is in alignment with a base end of dielectric strip line203 in straight line. Here, the transmission mode of electromagneticwaves to be transmitted through the dielectric strip line 203 is a LSEmode.

In FIG. 24, the line substrate 210 is mounted such that it is invertical relationship with respect to the parallel plates 201, 202. Thisembodiment presents an advantage in that high-order modes are hardlyraised in the signal line 212. It should be noted that it is alsopossible to employ an arrangement as shown in FIG. 23 in that thedielectric strip line 203 is in alignment with the signal line 212 andthat the line substrate 210 is mounted in a vertical manner with respectto the parallel plates 201, 202.

Embodiment 7

FIG. 25 is a view showing an arrangement of a NRD guide Gunn oscillatoraccording to the seventh embodiment of the present invention. The NRDguide circuit is arranged in that a dielectric strip line 203 is pinchedbetween two metallic parallel plates 201, 202 and is thus ofconventional arrangement. In the present embodiment, two Gunn diodes 220are mounted on a line substrate 210 that are supported with respect tothe flat plate 202 via heat sink 230 arranged for grounding and heatdissipation purposes.

The line substrate 210 is arranged, as shown in FIGS. 26(a) and (b), byforming onto an upper surface of a semi-insulating or insulating platesubstrate 211 (which may, for instance, be of AlN, Si, SiC or diamondhaving a resistivity of not less than 10⁶ Ωcm and a thermal conductivityof not less than 140 W/mK), a signal line 212, a choke portion 213 forapplying direct-current bias onto the signal line 212, two signalelectrodes 214 connected to both end portions of the signal line 212,and two pairs of surface ground electrodes 215 disposed as to pinch thetwo signal electrodes 214 between. A ground electrode 216 is formed on arear surface of the substrate, and the surface ground electrodes 215 areconnected to the ground electrode 216 through via holes 217. The linesubstrate 210 does not comprise ground electrodes on the rear surface ofthe signal line 212 and forms a suspended microstrip line.

The Gunn diode 220 is formed, as shown in FIGS. 27(a) and (b), bylaminating onto an upper surface of a semiconductor substrate 221 afirst contact layer 222, an active layer 223, a second contact layer 224and a metal layer 225, wherein a circular concave portion 226 is formedsuch that it substantially reaches from the metal layer 225 to the firstcontact layer 222. With this arrangement, the metal layer 225 issub-divided into anode electrode 225A and cathode electrode 225K, and abump 227 of Au that is easy to be bonded through thermo-compression isformed on the anode electrode 225A and a bump 228 similarly of Au on thecathode electrode 225K, such that their heights are respectively of samelevel. These bumps 227, 228 are equivalent to anode electrode 225A andcathode electrode 225K respectively. In one example, the semiconductorsubstrate 221 may be of n-type gallium arsenide having an impurityconcentration of 1 to 2×10¹⁸ atom/cm³, a first contact layer 222 ofn-type gallium arsenide having an impurity concentration of 2×10¹⁸atom/cm³ and a thickness of 1.5 μm, an active layer 223 of n-typegallium arsenide having an impurity concentration of 1.2×10¹⁶atom/cm³and a thickness of 1.6 μn, and a second contact layer 224 of n-typegallium arsenide having an impurity concentration of 1×10¹⁸ atom/cm³ anda thickness of 0.3 μm. It is also possible to employ an alternativecompound semiconductor such as indium phosphide instead of galliumarsenide. In the Gunn diode 220, an area of the sub-divided portion ofthe active layer corresponding to the anode electrode 225A is set to bean area with which a specified operating current of the Gunn diode canbe obtained (transverse cross section)

Further, an area of the active layer corresponding to the cathodeelectrode 225K is set to be not less than ten times as large as the areaof the active layer corresponding to the anode electrode 225A, and theelectric resistance of a semiconductor laminated portion below thecathode electrode 225K is made to be not more than {fraction (1/10)} ofthat of a semiconductor laminated portion below the anode electrode225A. With these arrangements, this portion is not made to function as aGunn diode but to function as a resistance of substantially low value.

It should be noted that the Gunn diodes 220 may be alternativelyarranged, as shown in FIG. 27(c), to be Gunn diodes 220′ in which thesecond contact layer 224 and active layer 223 underlying the cathodeelectrode 225K of FIG. 22(b) are omitted, wherein the cathode electrode225K is directly adhered to the first contact layer 222 and bumps 228thereof are provided to be of the same height levels as the bump 227 ofanode electrode 225A.

Assembly and mounting each of the Gunn diodes 220 to the plate substrate211 of the line substrate 210 is performed in that the bump 227 of theanode electrode 225A is connected to the signal electrode 214 and thepair of bumps 228 of the cathode electrodes 225K to the pair of surfaceground electrodes 215 through thermo-compression bonding. By making theportion of the ground electrodes 216 of the line substrate 210 beconnected to the heat sink 230, they are grounded to the flat plate 202through this heat sink 230. The other Gunn diode is similarly assembled.

Assembly of the line substrate 210 to the NRD guide circuit isperformed, as shown in FIG. 25, in that the plate substrate 211 of theline substrate 210 is made vertical with respect to parallel plates 201,202, and in that the center portion of the signal line 212 approachesthe base portion of the dielectric strip line 203 from a verticaldirection.

In case direct-current is applied to the choke portion 213, current issupplied through the signal electrode 214 to the Gunn diode 220 that iscloser to the choke 213 and via the signal electrode 214 through thesignal line 212 to the Gunn diode 220 that is remote from the chock 213,and via holes 217, ground electrode 216 on the rear surface, heat sink230, and plate 202 in this order, whereby electromagnetic waves(microwaves) are generated at the two Gunn diodes 220. The generatedelectromagnetic waves are resonated at the signal line 212 and a partthereof is combined with the dielectric strip line 203 for transmission.

Since the choke portion 213 is formed on the plate substrate 211 in thisembodiment, the choke portion can be formed simultaneously with thesignal line 212, signal electrode 214, and surface ground, electrodes215 through etching, whereby simple fabrication is enabled without theneed of removal of the substrate to improve efficiency of assembling.

Further, since the Gunn diode 220 is directly mounted onto the platesubstrate 211 in a face down posture, no parasitic inductance isgenerated unlike cases in which ribbons are used.

Since heat generated at the Gunn diodes 220 is transmitted to the heatsink 230 through bumps 227, 228 and the plate substrate 211 thatpresents high thermal conductivity, the heat dissipating effect can beimproved. Also, since the Gunn diode 220 is supported by the bumps 228of the cathode electrodes 225K from both sides, it can be prevented thatexcessive load is applied to the semiconductor laminated portion in thecenter that substantially functions as the Gunn diode.

Further, while the signal line 212 of the line substrate 210 is asuspended microstrip line in the abovedescribed example, the groundelectrode 216 may be provided on the whole rear surface of the platesubstrate 211 to make up a microstrip line. Such a line mayalternatively be a coplanar waveguide in which a signal line is providedin the center of the upper surface of the plate substrate 211 and a pairof ground electrodes are provided on the same plane as to pinch thesignal line between. In this case, the bump 227 of the anode electrode225A shall be connected to the signal line in the center and the bumps228 of the cathode electrodes 225K on both sides to the groundelectrodes.

Further, the anode electrode 22SA and cathode electrode 225K of the Gunndiode 220 may be reversed, depending on the concentration gradient ofthe active layer, and in such a case, the polarity of voltage applied tothe choke portion 213 shall be suitably selected.

FIG. 28 is a view in which the line substrate 210 is mounted such thatit is parallel with respect to the parallel plates 201, 202.

As explained so far, since etching for defining a region that is tofunction as a Gunn diode is performed by self-alignment dry etchingutilizing electrode layers formed above this region as masks, variationsin characteristics are restricted in the Gunn diode of the presentinvention.

Further, since the cathode electrode and anode electrode can be formedon a same plane to assume identical height levels in the Gunn diode ofthe present invention, the Gunn diode can be assembled in a face downposture. With this arrangement, the Gunn diodes do not need to beincorporated into conventional pill-type packages, whereby advantages interms of fabrication are presented to enable easy assembly to platesubstrates

Also, since it is not required to connect the Gunn diode to minuteelectrodes through means such as gold ribbons at the time of assembly,parasitic inductance do not occur and variations in circuitcharacteristics owing to variations in lengths of the gold ribbon or thelike can be eliminated.

Further, the arrangement of a plurality of individual mesa-typestructured portions that substantially function as a Gunn dioderemarkably improves the heat dissipating efficiency, and the conversionefficiency or output power can be largely improved.

In case of an assembly arranged with an oscillator, a portion of theoscillator is shielded by the Gunn diode or, in addition thereto, by aconductive plate substrate, phase noise can be largely decreased and theQ increased.

Further, connection for bias applying between the choke and the Gunndiode is made simple to enable simple fabrication whereby efficiency ofoperation is improved. No ribbon is required for mounting the Gunndiode, whereby generation of parasitic inductance can be prevented.Additionally, heat generated at the Gunn diode is transmitted to theheat sink through the substrate, heat dissipating effects can beimproved.

Also, connection for bias applying between the choke and the Gunn diodeis made simple to enable simple fabrication whereby efficiency ofoperation is improved.

No ribbon is required for mounting the Gunn diode, whereby generation ofparasitic inductance can be prevented,

Additionally, heat generated at the Gunn diode is transmitted to theheat sink through the substrate, heat dissipating effects can beimproved.

What is claimed is:
 1. A Gunn diode which is formed by sequentially laminating a first semiconductor layer, an active layer and a second semiconductor layer onto a semiconductor substrate, comprising: a first electrode and a second electrode arranged on the second semiconductor layer in a same plane for applying voltage on the active layer, the first electrode being surrounded by the second electrode; and a concave portion which is cut from a boundary between the first and second electrodes in a direction vertically toward the second semiconductor layer and the active layer and which subdivides the second semiconductor layer and the active layer to which the first electrode is connected as a region which functions as a Gunn diode.
 2. The Gunn diode of claim 1, wherein conductive protrusions are formed on the first and second electrodes successively to the underlying electrodes such that their upper surfaces assume a substantially identical level height.
 3. The Gunn diode of claim 2 wherein the conductive protrusion of the first electrode is formed substantially in a central portion and in that the conductive protrusions of the second electrode are formed at both sides thereof.
 4. The Gunn diode of claim 1 or 2, wherein there are provided at least two first electrodes and concave portions which have been cut from a boundary between the first and second electrodes.
 5. A Gunn diode which is formed by sequentially laminating a first semiconductor layer, an active layer and a second semiconductor layer onto a semiconductor substrate comprising: a first electrode and a second electrode arranged on the second semiconductor layer in a same plane for applying voltage on the active layer, the first electrode being surrounded by the second electrode; and a concave portion which is cut from a boundary between the first and second electrodes in a direction vertically toward the second semiconductor layer and the active layer and which subdivides the second semiconductor layer and the active layer to which the first electrode is connected as a region which functions as a Gunn diode, wherein an area for the first electrode is set to be not more than {fraction (1/10)} of an area for the second electrode.
 6. The Gunn diode of claim 5, wherein conductive protrusions are formed on the first and second electrodes successively to the underlying electrodes such that their upper surfaces assume a substantially identical level height.
 7. The Gunn diode of claim 6, wherein the conductive protrusion of the first electrode is formed substantially in a central portion and in that the conductive protrusions of the second electrode are formed at both sides thereof.
 8. The Gunn diode of claim 5 or 6, wherein there are provided at least two first electrodes and concave portions which have been cut from a boundary between the first and second electrodes. 